Scientific Method —

Dark matter stars may have lit the early universe

New research shows that stars made of dark matter, "dark stars" as the …

New work from a team of physicists sheds light—bad pun intended—on the formation of the first stars in the universe. The unexpected twist? Dark matter could have played a major role in the formation of these "dark stars." Where conventional stars rely on fusion of light elements into heavier ones to counteract the astronomical gravitational forces acting on such a massive body, dark stars are held up by the annihilation reaction that occurs when dark matter comes into contact with antimatter.

As the article, set to be published in an upcoming edition of Physical Review Letters, begins, "The first stars in the universe mark the end of the cosmic dark ages." These earliest stars began life inside dark matter halos that consisted of approximately 85 percent dark matter and 15 percent baryonic matter, the latter mostly in the form of hydrogen and helium. The baryonic matter would cool and coalesce into a small protostar at the center of the dark matter halo. The authors ask the question: what role did the surrounding dark matter play in the formation of these early stars?

In order to model the dark matter stars, the physicists needed to decide on a dark matter particle model. Since this is still a wide-open issue in physics, they used a particle known as the neutralino, the supersymmetric counterpart of the W and Z bosons. The neutralino has the correct mass—in the GeV to TeV range—which can account for the proper amount of dark matter believed to be in the universe today.

To understand the formation of dark stars, the researchers adiabatically contracted an initial mixture of dark matter and normal gas and simulated the evolution of the protostar. As the matter contracts, the core begins to heat up. This happens not as a result of a traditional stellar fusion reaction, but is due to a matter-antimatter annihilation reaction.

Early on in the process, the density of the cloud is still quite low and the heat due to the reaction is simply radiated away. As the density increases, a critical point is reached where the majority of the annihilation energy is trapped inside the core. At this point the heat and pressure make further collapse difficult.

In addition to heating of the core by dark matter annihilation, there are multiple cooling effects that are taken into account. The dominant mode of cooling comes from the formation of molecular hydrogen from atomic hydrogen. This combination of effects results in a competition between heating and outward pressure from dark matter annihilation, and cooling and gravitational collapse.

Bullet cluster showing a separation of normal and dark matter

With the conclusion that the formation and life cycle of a population III star can be drastically altered by the dark matter annihilation reaction within the protocloud, the authors ask what the lifetime of such an object might be. Using the values found throughout the paper the authors estimate that the dark stars would last about as long as the dark matter annihilation reaction would proceed, on the order of 10 million to 100 million years.

With the predictions made, the authors put forth a series of observations/calculations that can be made to support or refute their ideas. These include looking at the reionization of the intergalactic medium, examining at the mass of population III stars, and potentially even seeing them using the James Webb Space Telescope. If the dark stars shine at lower temperatures, the Webb Telescope could find them and differentiate their spectra from those expected from the traditional first star formation scenarios. This letter presents an interesting idea that could shed further light into the nature of both dark matter, and the early universe.

Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm